This invention relates to high density ion exchange resins, particularly to a method for preparing high density ion exchange resins from a cross-linked, aromatic polymer.
Ion exchange resins are normally solid materials which have the ability to exchange or absorb ions in a liquid without the resin exhibiting substantial structural alteration. For this reason, the resins, conventionally in the form of spheroidal beads, are widely employed in a variety of continuous ion exchange operations such as waste treatment, e.g., the continuous removal of undesirable components from water, and recovery processes such as the recovery of uranium. In such continuous operations, particularly up-flow or fluidized bed operations, to effectively remove the ionic ingredients from solution, the ion exchange resin advantageously possesses a density or other property sufficient to prevent entrainment losses and to provide intimate and constant contact between the resin and the ion containing liquid which is often a thick pulp or slurry.
Conventionally, many ion exchange resins are prepared by the suspension polymerization of a mono-vinylidene aromatic, such as styrene, with a polyvinylidene aromatic such as divinylbenzene, followed by the attachment of ion-active exchange groups to the resulting cross-linked aromatic polymer. For example, anion exchange resins are conventionally prepared by halomethylating and subsequently aminating the polymer. See, for example, Ion Exchange by F. Helfferich, published in 1962 by McGraw-Hill Book Company, New York. Unfortunately, in many ion exchange operations, particularly operations such as uranium recovery wherein the ion containing liquid is a thick slurry, conventionally prepared ion exchange resins have a sufficiently low density that the resins tend to float in the ion containing liquid, thereby limiting their effectiveness.
Various methods have been proposed for increasing the density of an ion exchange resin. For example, U.S. Pat. Nos. 2,769,788 and 2,809,942 disclose the incorporation of an inert, finely divided, solid material having a high density, i.e., 2.5 g/cc or higher, into a copolymer bead of monovinylidene and polyvinylidene aromatics. Unfortunately, such beads exhibit surface irregularities, excessive spalling and low mechanical stability, thereby tending to break when employed in a continuous operation.
Alternatively, German Pat. No. 2,218,126 proposes preparing a high density ion exchange resin using a nonionic substituted styrene, e.g., monochlorostyrene. Unfortunately, nonionic substituted styrenes are generally relatively expensive and of limited availability.
Recently, as described in Belgian Pat. No. 858,142, an anion exchange resin having increased density was prepared from a chloromethylated, cross-linked copolymer of styrene which has been halogenated in the presence of a Friedel-Crafts catalyst. However, the anion exchange resin prepared by such method generally exhibits excessive cracking or breaking and reduced water holding capacity. In addition, the separation of the catalyst from the halogenated copolymer and subsequent disposal of the resulting wash liquor in an environmentally acceptable manner is not readily accomplished.
In view of the stated deficiencies in the prior art methods for preparing ion exchange resins having higher densities, it would be highly desirable to provide an improved method for preparing such resins.